A variety of military and commercial applications including communications, sensing and vehicle hubs require scalable, reliable power grid based on power branching units (PBUs). When these applications include PBUs in remote and hard-to-access areas, such as under see, rugged mountains, or rural areas, the PBUs require a very long period between service time and thus need to be reliably protected from electromagnetic pulse (EMP)-type surges caused by cable breakdowns, hostile environment, etc. Typically, these systems primarily operate from a constant current source remotely located from the PBUs. Such systems, where a single trunk (backbone) cable feeds many loads in series, may extend to hundreds or thousands of miles.
An EMP can occur as a radiated, electric or magnetic field or a conducted electric current, depending on the source. EMP interference is generally disruptive or damaging to electronic equipment, and at higher energy levels a powerful EMP event, such as a lightning strike, a cable breakdown, can damage physical objects, such as the PBUs. For example, any cable breakdown interrupts the current and releases the energy stored in the cable and therefore creating an equipment-damaging power surge. Electronic clamps internal to PBUs are incapable of sustaining power surges in the range of hundreds of kilowatts. Moreover, conventional clamping protectors, for example, metal oxide varistors (MOVs) do not have adequate reliability and volumetric characteristics.
Conventional approaches apply an emergency crowbar circuit across the PBUs for the surge duration, where the (surged) current flows through the crowbar bypassing the PBUs during surges. Power crowbar circuits typically use a large vacuum tube (i.e. Ignitron) or a thyristor to short the current path. However, vacuum tubes are large and expensive, while thyristors are smaller and offer lower cost, but they latch when triggered and stay on until the power is removed. When latched, thyristors or silicon controlled rectifier (SCR)-based circuits are not capable of removing the short circuit when the power surge is over. Also, transistor-base circuits lack the overcurrent capabilities of SCR-forced-commutated circuits using gate turn off (GTO) thyristors and have to operate with reduced reliability for high voltage and/or current rates (dv/dt and di/dt). In other words, when the crowbar is triggered, stored energy in the system has to be dissipated, while the surge current being dissipated is not controlled. However, the crowbar shall not interrupt the current flow in constant-current systems. The advantage of a crowbar circuit over a clamp circuit is that the low holding voltage of the crowbar circuit lets it carry higher fault current without dissipating much power (which could otherwise cause overheating). Also, a crowbar circuit is more likely than a clamp circuit to deactivate a device (by blowing a fuse or tripping a breaker), bringing attention to the faulty equipment.
FIG. 2 shows a typical crowbar circuit 200. As shown, an adjustable Zener diode (regulator) 202 controls the gate of a bidirectional triode thyristor (TRIAC) 204. The R1 and R2 resistor divider provides a reference voltage for the Zener diode 202. The R1 and R2 resistor divider is set so that during normal operating conditions, the voltage across R2 is slightly lower than VREF of the Zener diode 202. Since this voltage is below the minimum reference voltage of the Zener diode, the Zener diode remains off and thus very little current is conducted through the diode and its cathode resistor 206. If the cathode resistor 206 is sized accordingly, very little voltage will be dropped across it and the TRIAC gate terminal 208 will be at the same potential as main terminal 109 of the TRIAC, keeping the TRIAC off. If the supply voltage 210 increases, the voltage across R2 will exceed VREF and the Zener diode 202 begins to regulate the voltage, drawing more current through it. The voltage at the gate terminal 208 is then pulled down to the Zener diode voltage, exceeding the gate trigger voltage of the TRIAC and latching it on. When the crowbar is activated, the surge current deactivates the device (e.g., a PBU) by blowing a fuse 212 or tripping a breaker.
Once a crowbar circuit is triggered, it pulls the voltage below the trigger level, typically close to ground voltage level. Accordingly, a crowbar circuit does not automatically return to normal operation when the overvoltage condition is removed, unless the power is removed entirely to stop its conduction. In contrast, a clamp circuit prevents the voltage from exceeding a preset level.